This invention pertains to methods and devices for controlling the propagation of sound and particularly to electromagnetically tunable acoustic devices.
Propagation of sound waves through various media has been studied for centuries. Propagation of sound waves through periodic media dates back to the late 1800's and the word of Gerhard Floquet. The groundwork for understanding wave propagation in three-dimensional periodic media as it is currently understood was established by Felix Bloch in 1928. The Bloch-Floquet theorem describes how a wave can travel through a periodic medium without scattering. Using the Bloch-Floquet theorem, developments in electronics, which deal primarily with the flow of electrons through a structure, and photonics, which deals the propagation of photons through a periodic structure, were made. Especially important was the development of theories to create electronic and photonic bandgaps. The theoretical and experiment development of photonic bandgaps lead directly to the development of a theory for phononic bandgap structure.
A phonon is a quantized vibration of a material analogous to the photon being a quantized oscillation of an electromagnetic field. Sound is a vibration of air and can thus be described in phononic terms. For example, sound is an audible vibration of air, and can thus be quantified as phonons. Earthquakes are non-audible vibrations of Earth's crust, and can similarly be quantified as phonons. The vibrations felt while driving a car are vibrations of the material structure of the car and can thus be characterized and described by phonons. Any vibration of a medium, whether audible, mechanical, or otherwise can be described by a phonon. Acoustics is the generalized term used for the behavior of any type of phonon. Thus the acoustic behavior of a tuning fork would characterize the sound emitted by the tuning fork, and how it vibrates and responds to vibrations. The acoustic behavior of a bridge would characterize the response of the bridge to vibrations including what types of vibrations could cause the bridge to collapse. The dynamics of the propagation of phonons through structures can be determined by applying the appropriate version of the wave equation. A bandgap is a range of phonon frequencies where no phonons can be transmitted through a material. A material exhibiting phononic bandgap behavior is also referred to as a phononic crystal.
Whereas photonic waves possess only a transverse component, phononic waves can have both longitudinal and transverse components. Using similar techniques to those used for photonic crystal, a structure exhibiting an acoustic bandgap could be made.
Tunable phononic crystals were first theoretically presented in 2003 (Khelif et al. 2003). The first tunable phononic crystal was tuned by physically changing the size of scatterers in the lattice. Tuning of a bandgap by changing the physical dimensions of the structure is difficult in practice. Physical tuning can result in unwanted defects in the lattice that would modify the bandgap or path of sound in the phononic crystal. In recent years, other methods for tuning phononic crystals have been introduced including electric (Tang and Lee 2007) or magnetic fields (Robillard et al. 2009), rotation of the crystal (Goffaux and Vigneron 2001), or by physically combining or taking apart two periodic structures (Wang et al. 2009).
Ideally, a method for tuning photonic crystal should be developed which does not require physical contact.